Question

Which of the following amino acids does not have an \(L\) enantiomer? A. Cysteine B. Threonine C. Glutamic acid D. Glycine

Short answer

D. Glycine

Step-by-step solution

Understand Enantiomers

Enantiomers are molecules that are mirror images of each other but cannot be superimposed. This property is common in molecules that have a chiral center.

Identify Chirality in Amino Acids

Most amino acids have a chiral center at the alpha carbon, which is bonded to four different groups: an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group).

Analyze Each Amino Acid

Examine each provided amino acid to determine if it has a chiral center at the alpha carbon: A. Cysteine: Has a chiral center B. Threonine: Has a chiral center C. Glutamic acid: Has a chiral center D. Glycine: No chiral center since the R group is a hydrogen atom.

Conclude Based on Chirality

Since Glycine does not have a chiral center, it cannot have an enantiomer, including the specific case of not having an L enantiomer.

Key concepts

Enantiomers

Enantiomers are a fascinating concept in chemistry. They are pairs of molecules that are non-superimposable mirror images of each other. Think of your left and right hands: they are mirror images but cannot be perfectly aligned on top of each other. This unique property primarily arises in molecules with a chiral center. Enantiomers are crucial in biochemistry because they can have different biological activities. For instance, one enantiomer of a drug might be therapeutic, while the other could be harmful.

To better understand enantiomers, it helps to consider chiral centers and how they contribute to the unique properties of these mirror-image molecules.

Chiral Center

A chiral center is a carbon atom bonded to four different groups. This arrangement makes the carbon atom asymmetric, leading to the potential for chirality. In amino acids, the alpha carbon is usually the chiral center because it is bonded to:

• An amino group (-NH2)
• A carboxyl group (-COOH)
• A hydrogen atom (H)
• A side chain (R group)

If all these groups are different, the molecule can exist in two non-superimposable forms (enantiomers), typically designated as L and D forms.

For example, the amino acid cysteine has a chiral center at the alpha carbon, making it capable of having enantiomers. Not all amino acids have a chiral center, however, which is essential when considering glycine, as discussed next.

Glycine Chirality

Glycine is unique among the standard amino acids because it does not have a chiral center. The side chain (R group) of glycine is a hydrogen atom (H). Due to this, the alpha carbon in glycine is bonded to two hydrogen atoms, not four different groups. This lack of a chiral center means glycine cannot form enantiomers.

Therefore, glycine does not have an L or D form. In the context of the provided exercise question, glycine stands out as the one amino acid that does not have an L enantiomer due to its achirality. Understanding this distinction is fundamental to grasping why only glycine lacks the characteristic L enantiomer among common amino acids.